1. Field of the Invention
The present invention relates to a battery pack of an assembled battery composed of a plurality of laminated cells sheathed with a laminate film. In particular, the present invention relates to a battery pack of an assembled battery of a structure mechanically secured against an external force such as vibration or impact exerted on the battery, and also relates to a fixing method for the battery.
2. Related Art
A conventional battery pack is composed of one cell. Such a battery pack is of a small capacity and its use is often limited to applications involving relatively little vibration or impact. In recent years, a lightweight and compact, yet high-capacity, battery composed of a plurality of cells, such as a lithium battery, has been developed for use in portable devices, electric automobiles, and the like. Herein, the assembled battery may be called merely “battery”.
Each individual cell (herein, a battery cell provided with a pair of positive and negative electrode terminals and constituting the minimum output unit of an assembled battery is referred to as the cell) of a lithium battery or the like used for such applications is constructed as follows.
That is, the electrode terminal portion of a flat-shaped cell is sandwiched between laminate films for maintaining sealing property through the intermediation of a sealant such as polyethylene, followed by heat sealing along the outer peripheral edge portion to thereby prevent leakage of electrolyte.
Since an assembled battery used in portable devices or electric automobiles is required to have a large capacity, the battery uses a large number of laminated cells, so that the mass of the battery pack becomes large.
Further, since such a battery is used under environments involving a larger vibration amplitude and a broader vibration frequency band than those of conventional batteries, it is expected that fatigue breaking in the electrode terminal portion of each cell, the welding connection portion between the electrode terminals of cells, and the welding connection portion between the electrode terminals of cells and the battery terminals of the obtained battery, or breakage of the heat-seal portion of each cell can readily occur.
In this view point, with respect to a battery pack of an assembled battery having a structure in which a plurality of such flat-shaped cells are laminated so as to obtain a predetermined output, there has been proposed a technique according to which the laminate of the individual cells of the battery is fixed in place by fixing means composed of a pressing member pressing the surface of sealing means, and an elastic member such as a spring which supports and fixes the pressing member in place, thereby preventing, with respect to vibration applied from the outside of the battery pack, fatigue breaking of the welding connection portion of the electrode terminals and leakage of an electrolyte solution due to peeling of the heat-seal portion of each cell as disclosed, for example, Japanese Patent Application Laid-open Publication No. 2004-55346.
The fixing means disclosed in this Japanese publication employs a vibration suppression method by which the centroid position of the contact surface between the cells and the pressing member is supported and pressed by an elastic member such as a spring. The fixing means is thus a fixing method for suppressing natural vibration of the cells.
Accordingly, since the laminated cells are supported by an elastic member, displacement due to the vibration constantly occurs, so that such problems as positional displacement of respective cells, fatigue breaking of the welding connection portion of the electrode terminals, and breakage of the heat-seal portion of the cells will still occur.
Further, the above-described method of supporting the centroid position by an elastic member involves problems in that the structure becomes complicated due to the increased number of parts for the fixing means, and that the size of the battery pack of the battery is enlarged because of the increased dead space due to the necessity of securing a space for allowing displacement of the spring as the elastic member.